PETE 203: Properties of oil types of non-hydrocarbon compounds occur in crude oils and refinery...
Transcript of PETE 203: Properties of oil types of non-hydrocarbon compounds occur in crude oils and refinery...
PETE 203: Properties of oil
Prepared by: Mr. Brosk Frya Ali
Koya University, Faculty of Engineering,
Petroleum Engineering Department
2013 – 2014
Lecture no. (2): Crude oil –chemistry and composition
The crude oil mixture is composed of the following groups:
1. Hydrocarbon compounds
2. Non hydrocarbon compounds
3. Organo-metallic compounds and inorganic salts
5. Crude oil Chemistry
The chemical properties (physical also) of crude oil depends
on its unique mixture of molecule.
5.1 Hydrocarbon compounds:-
Hydrocarbons may be gaseous, liquid, or solid at normal temperature
and pressure, depending on the number and arrangement of the
carbon atoms in their molecules.
Those with up to 4 carbon atoms are gaseous; those with 20 or more
are solid; those in between are liquid.
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All hydrocarbon classes are present in the crude mixture, except
alkenes and alkynes.
Alkanes (paraffins) are saturated hydrocarbon. They have only single
bonds such as CH4, C2H6, C3H8, with general formula CnH2n+2.
The naming according to IUPAC ended with ane which means single.
Alkenes (Olefins) are unsaturated hydrocarbon. They have double
bonds such as Ethene (Ethylene) C2H4, propene C3H6, butene C4H8
with general formula CnH2n.
The naming according to IUPAC ended with ene which means double.
Alkynes are unsaturated hydrocarbon. They have triple bonds such as
Ethyne (acetylene) C2H2, propyne C3H6, with general formula CnH2n - 2.
The naming according to IUPAC ended with yne which means triple.
The principal constituents of most crude oils are hydrocarbon
compounds.
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5.1.1 Alkanes (paraffins)
The following is a brief description of the different hydrocarbon
classes found in all crude oils.
Methane, Ethane, propane and butane are gaseous hydrocarbons at
ambient temperatures and atmospheric pressure. They are usually
found associated with crude oils in a dissolved state.
A branched alkane with the same number of carbons and hydrogen
as an n-alkane is called an isomer.
Normal alkanes (n-alkanes, n-paraffins) are straight-chain
hydrocarbons having no branches.
Branched alkanes are saturated hydrocarbons with an alkyl
substituent or a side branch from the main chain.
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For example, butane (C4H10) has two isomers, n-butane and
2-methyl propane (isobutane).
As the molecular weight of the hydrocarbon increases, the
number of isomers also increases.
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Pentane (C5H12) has three isomers; hexane (C6H14) has five.
The following shows the isomers of hexane:
Crude oils contain many short, medium, and long-chain normal
and branched paraffins.
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5.1.2 Cycloparaffins (Naphthenes)
Saturated cyclic hydrocarbons, normally known as naphthenes,
are also part of the hydrocarbon constituents of crude oils. Their
ratio, however, depends on the crude type.
The cycloalkanes contain only single bonds, and have the
general formula CnH2n.
Cyclomethane and cycloethane obviously cannot exist, but
cyclopropane, cyclobutane, cyclopentane etc can exist.
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Cyclohexanes, substituted cyclopentanes, and substituted
cyclohexanes are important precursors for aromatic
hydrocarbons.
The examples shown here are for three naphthenes of special
importance.
The lower members of naphthenes in crude oil are
cyclopentane, cyclohexane, and their mono-substituted
compounds.
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If a naphtha fraction contains these compounds, the first two
can be converted to benzene C6H6, and the last compound can
dehydrogenate to toluene C7H8 during processing.
Heavier petroleum fractions such as kerosine and gas oil may
contain two or more cyclohexane rings bonded through two
neighboring carbons.
The content of cycloparaffins in petroleum varies up to 60% of
the total hydrocarbons.
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5.1.3 Aromatic Compounds
Aromatic hydrocarbons are unsaturated hydrocarbons which
have one or more benzene ring, to which hydrogen atoms are
attached according to the formula CnHn.
Benzene ring is hexagonal ring arrangement found in benzene
and other aromatic compounds, consisting of six carbon atoms
with alternating single and double bonds between them, and
with each carbon atom bonded to a hydrogen atom, or to other
atoms or groups of atoms in derivatives of benzene.
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Toluene (C7H8) and xylene (C8H10) are also mononuclear aromatic
compounds found in variable amounts in crude oils.
Benzene, toluene (C7H8), and xylenes (C8H10) (BTX) are
important petrochemical intermediates as well as valuable
gasoline components.
Separating BTX aromatics from crude oil distillates is not
feasible because they are present in low concentrations.
Enriching a naphtha fraction with these aromatics is possible
through a catalytic reforming process.
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5.2 Non-hydrocarbon
Compounds
The table shows the percentage
by weight of hydrocarbons in the
crude oils
Table 2.1 Composition by weight of hydrocarbons in the crude oil
Hydrocarbons Average
Paraffins 30 %
Napthenes 49 %
Aromatics 15 % Asphaltic 6 %
The presence of these impurities is harmful and may cause
problems to certain catalytic processes. Fuels having high sulfur
and nitrogen levels cause pollution problems in addition to the
corrosive nature of their oxidization products.
Various types of non-hydrocarbon compounds occur in crude
oils and refinery streams. The most important are the organic
sulfur, nitrogen, and oxygen compounds. Traces of metallic
compounds are also found in all crudes.
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5.2.1 Sulfur Components
Sulfur in crude oils is mainly present in the form of organo-sulfur
compounds. Hydrogen sulfide is the only important inorganic
sulfur compound found in crude oil.
Its presence, however, is harmful
because of its corrosive nature and
it can kill an operator in 10 seconds
(at concentration 1000 ppm)
Fortunately sulphides have a highly obnoxious (horrible) smell
which gives some warning of their danger.
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Acidic sulfur compounds are the thiols (mercaptans). Thiophene,
sulfides, and disulfides are examples of non-acidic sulfur
compounds found in crude fractions.
Organosulfur compounds may generally be classified as acidic
and non-acidic.
If both of the two hydrogen atoms are replaced by hydrocarbon
groups, the compound is called a sulfide or thioether.
Mercaptans are acidic sulfur compounds .
If one of the hydrogen atoms is replaced by a hydrocarbon group,
the compound is called a mercaptan or thiol. Such compounds are
formed during the distillation of crude oils.
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Examples of some sulfur compounds from the two types are:
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Sour crudes contain a high percentage of hydrogen sulfide.
Because many organic sulfur compounds are not thermally
stable, hydrogen sulfide is often produced during crude
processing.
High-sulfur crudes are less desirable because treating the
different refinery streams for acidic hydrogen sulfide increases
production costs.
The presence of sulfur compounds in finished petroleum
products often produces harmful effects.
In addition, mercaptans in hydrocarbon solution cause the
corrosion of copper and brass in the presence of air.
For example, in gasoline, sulfur compounds promote corrosion
of engine parts.
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Gasoline with a sulfur content between 0.2 and 0.5% has been
used without obvious harmful effect.
In diesel fuels, sulfur compounds increase wear and can
contribute to the formation of engine deposits.
Most sulfur compounds are removed from petroleum streams
through hydro-treatment processes (Hydrodesulphurization)
In this process hydrogen sulfide is produced and the
corresponding hydrocarbon released. Hydrogen sulfide is then
absorbed in a suitable absorbent and recovered as sulfur.
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5.2.2 Nitrogen Components
Organic nitrogen compounds occur in crude oils either in a simple
heterocyclic form as in pyridine (C5H5N) and pyrrole (C4H5N), or
in a complex structure as in porphyrin.
Porphyrins are a group of organic compounds which occur in
nature. One of the best known is heme, the pigment in red blood
cells.
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Nitrogen compounds in crude oils are complex and distillation
may give rise to nitrogen compounds.
The nitrogen content in most crudes is very low and does not
exceed 0.1 % .In some heavy crudes, however, the nitrogen
content may reach up to 0.9 % wt.
Nitrogen compounds are more thermally stable than sulfur
compounds and accordingly are concentrated in heavier
petroleum fractions and residues.
Nitrogen has to be removed from crude oil because
• Nitrogen impurities in hydrocarbon fuels have a severe
environmental impact resulting from the contribution
nitrogen oxides (produced during combustion) to acid rain;
• Nitrogen impurities are effective catalyst poisons that slow
down the processing of crude oil.
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Nitrogen compounds are removed by hydrodenitrogenation to
ammonia. For example, pyridine is denitrogenated to ammonia
and pentane:
Nitrogen compounds in crudes may generally be classified into
basic and non-basic categories.
Basic nitrogen compounds are mainly those having a pyridine
ring, and the non-basic compounds have a pyrrole structure.
Both pyridine and pyrrole are stable compounds due to their
aromatic nature.
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The following are examples of organic nitrogen compounds.
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5.2.3 Oxygen Components
Oxygen compounds in the crude oils are more complex than the
sulfur types. However; their presence in petroleum streams is
not poisonous to processing catalysts.
Many of oxygen compounds found in crude oils are weakly
acidic. They are carboxylic acids -C(=O)OH or -COOH,
cresylic acid, phenol, and naphthenic acid.
Naphthenic acids are mainly cyclopentane and cyclohexane
derivatives having a carboxylic (-COOH).
Cresylic acid is any of several acids derived from petroleum
and coal tar that boil above 204°C, contain varying amounts of
xylene and cresol.
A cresol molecule has a methyl group substituted onto phenol
molecule.
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The total acid content of most crudes is generally low, but may
reach as much as 3%, as in some California crudes
Naphthenic acids in the naphtha fraction have a special
commercial importance and can be extracted by using dilute
caustic solutions.
Non acidic oxygen compounds such as esters, ketones, and
amide are less valuable than acidic compounds.
The following shows some of the oxygen compounds commonly
found in crude oils.
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Acidic Oxygen
Compounds
Non-Acidic
Oxygen
Compounds
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5. 3 Metallic Components
Many metals occur in the crude oils. Some of the more
abundant are sodium (Na), calcium (Ca), magnesium (Mg),
aluminum (Al), iron (Fe), vanadium (V) and nickel (Ni).
They are present either as inorganic salts, such as sodium and
magnesium chloride, or in the form of organometallic
compounds, such as those of nickel and vanadium.
Calcium and magnesium can form salts or soaps with
carboxylic acids. These compounds acts as emulsifiers and their
presence is undesirable.
Although metals in crudes are found in trace amounts, their
presence is harmful and should be removed.
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When crude oil is processed sodium and magnesium chlorides
produce hydrochloric acids, which is very corrosive.
Desalting crude oils is a necessary step to reduce these salts.
Vanadium and nickel are poisons to many catalysts and should
be reduced to very low levels.
Solvent extraction processes used to reduce the
concentration of heavy metals in petroleum residues.
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